Drop on demand ink jet technology is widely used in the printing industry. Printers using drop on demand ink jet technology may use a plurality (i.e., an array) of electrostatic actuators, piezoelectric actuators, or thermal actuators to eject ink from a plurality of nozzles in an aperture plate. In electrostatic ejection, each electrostatic actuator, which is formed on a substrate assembly, typically includes a flexible diaphragm or membrane, an ink chamber between the aperture plate and the membrane, and an air chamber between the actuator membrane and the substrate assembly. An electrostatic actuator further includes an actuator electrode formed on the substrate assembly. When a voltage is applied to activate the actuator electrode, the membrane is drawn toward the electrode by an electric field and actuates from a relaxed state to a flexed state, which increases a volume of the ink chamber and draws ink into the ink chamber from an ink supply or reservoir. When the voltage is removed to deactivate the actuator electrode, the membrane relaxes, the volume within the ink chamber decreases, and ink is ejected from the nozzle in the aperture plate.
One critical aspect of electrostatic actuators is the dimensions of a spacing or gap between the actuator electrode and the membrane. The gap affects both the volume of ink ejected from a nozzle upon removal of the voltage from the actuator electrode and the voltage that must be applied to the actuator electrode to sufficiently deflect the membrane. A gap that is too narrow or too wide will eject either an insufficient or excessive quantity of ink respectively. Further, as the gap height increases, the power that must be applied to the actuator electrode to sufficiently deflect the membrane also increases.
An electrostatic actuator further includes a dielectric gap standoff layer formed over the substrate assembly, and may be formed on portions of the conductive layer that is used to form the actuator electrodes. The membrane is adhered or bonded to an upper surface of the gap standoff to space the membrane from the electrode, and thus a thickness of the gap standoff layer partially determines the gap or spacing between the actuator electrode and the membrane, which is a critical dimension that affects operation of the printhead.
Additionally, an electrostatic actuator can include a body layer that overlies, and is attached to, the membrane and is used for mounting of the nozzle plate that includes a plurality of nozzles. Thus each ink chamber can be defined, at least in part, by the membrane, the body layer, and the nozzle plate.
For most efficient and predictable operation of a printhead, each electrostatic actuator is designed to have a membrane with a target width “WT”. The alignment of the body layer to the gap standoff layer in part determines an effective (i.e., operational or functional) width “WE” of the membrane for a particular electrostatic actuator. In a perfectly aligned printhead, the body layer is correctly aligned with the gap standoff layer, and the effective width WE is equal to the target width WT. When the body layer is correctly aligned with the gap standoff layer, the operational characteristics of the membrane, for example the flex and travel of the membrane during ejection of ink from a nozzle of the nozzle plate, are close to their designed values, and ink is ejected in the proper volume and direction of travel. FIG. 5A depicts an electrostatic actuator 500 of an electrostatic ink jet printhead where the body plate 502 is properly aligned to the gap standoff layer 504. When the body plate 502 is properly aligned to the gap standoff layer 504, the membrane 506 for the electrostatic actuator 500 has a target width of WT and an effective width of WE, where WE=WT.
It will be appreciated that each membrane 506 for each individual actuator 500 is formed from a continuous membrane layer that provides a membrane 506 for a plurality of actuators 500. The membrane or diaphragm 506 for each individual actuator 500 is the region that flexes between membrane nodes, wherein the nodes are provided by the individual gap standoff sections 504 and/or the individual body layer sections 502, depending on the alignment of the body layer 502. In FIG. 5A, the membrane nodes are provided by both the gap standoff layer 504 and the body layer 502, as the individual sections of these layers have the same width and are properly aligned.
In contrast, a body layer that is misaligned to the gap standoff layer decreases the effective width of the membrane for every actuator across the printhead. When the body layer is misaligned to the gap standoff layer, the operational characteristics of the membrane deviate from their designed values, and ink droplet volume and direction of travel may be adversely affected. FIG. 5B depicts an electrostatic actuator 510 that is part of an array of similar electrostatic actuators of an electrostatic ink jet printhead, where the body plate 512 is misaligned to the gap standoff layer 514. When the body plate 512 is misaligned to the gap standoff layer 514 as depicted, the membrane 516 for the electrostatic actuator 510 still has a target width of WT, but WE is decreased such that WE<WT. The flex and travel of the membrane 516 may be decreased which, in turn, may decrease the volume of the ejected ink droplet and adversely affect the trajectory of the ejected ink droplet, thereby decreasing print quality. In FIG. 5B, the membrane node is provided on the left side of the actuator 510 by the body layer 512, and on the right side of the actuator by the gap standoff layer 514, as the two layers are misaligned.
A method and structure for an electrostatically actuated ink jet printhead that has improved resistance to body layer misalignment and increases print quality, particularly in misaligned printheads, would be desirable.